(1) Field of the Invention
This invention relates to apparatus for stretching a moving web of material, and relates particularly but not exclusively to apparatus for stretching a moving web of plastic material, for example polyethylene and polypropylene.
(2) Prior Art and Technical Considerations
It is well known that certain plastic material webs can acquire improved physical properties by stretching either when the web is hot or when it is cold. It is also known that while such stretching is carried out, it is preferable that the web should be under physical control.
Various apparatuses for stretching webs in a controlled manner have been described. Of these, there is a type of apparatus which employs articulated chains. For example, there is described in British Patent Specification No. 936,965 such an apparatus which comprises two endless articulated chains each of which is driven and guided along a predetermined path, the chains being caused to expand as they travel along their working runs which extend on either side of the path of the web of material. A number of web gripping devices are connected at intervals to each chain and these, in operation, grip the edges of the web such that a pulling action in the axial and/or transverse direction is transmitted to the web. Longitudinal stretching of the web is achieved by expanding the chain thereby increasing the distance between adjacent gripping devices. Transverse stretching is achieved by arranging that over part at least of the working runs of the chains, the chains diverge with respect to the web.
British Patent Specifiction No. 1,091,971 describes a similar apparatus to that described in British Patent Specification No. 936,965 in which means are provided whereby a adjustments may be made to the stretch ratios in the longitudinal and transverse directions of a web while the apparatus is in operation.
In such apparatuses, the expansion and contraction of the chains and the paths of the folded chains are controlled by guiding each link at two points spaced thereon, each guide member associated with each of said points being in substantially constant engagement with guide means. In consequence, even when a chain is not being subjected to controlled expansion or contraction, the guide members in engagement with guide means, such as a track, are contributing to the total frictional drag on the movement of a chain. In addition, the bearings associated with the guide members are suffering undue wear. However, web stretching apparatuses based on the articulated chain principle have a number of advantages over other types. One particular feature is that in expanding the chain by a simple means, such as described in British Patent Specifications Nos. 936,965 and 1,091,971, the rate of expansion is automatically progressively reduced. This can be a desirable feature of an apparatus for use in the stretching of webs of plastic materials. Accordingly, this type of apparatus is of considerable importance and it is therefore desirable to obtain a high degree of efficiency for such apparatus. Towards this end, it is highly desirable to reduce the frictional drag and wear. In addition, it is also desirable to reduce to a minimum the proportion of the path of the chain in which the chain is in the folded state and especially in the almost completely folded state, while each of the guide members is in engagement with guide means, in which state of the chain the toggle action which may be set up when the chain is subjected to a compression force, and which may result in unsmooth running of the apparatus, may have its greatest effect. This toggle action may result particularly from a build-up friction arising between the guide members and the guide means tending to retard the progress of the chain in response to a compression force applied to the chain in the intended direction of motion of the chain. The normal running friction arising between the guide members or attachments thereto and the guide means may be undesirably increased through any tendency for the guide members or attachments thereto to become distorted or misaligned relative to guide means contacted thereby. As a result, it has been found to be of paramount importance to reduce to a low level the friction between the guide members or the attachments thereto and the guide means and to limit the possible distortion or misalignment, whether temporary or permanent, of the guide members relative to the guide means.